1. Field of the Invention
The present invention relates to packaging systems for transporting containers principally storing circular works such as semiconductor wafers or magnetic disks. The present invention particularly relates to a damper system for transportation. The damper system includes a packaging box, such as a corrugated fiberboard box, for storing a container containing a work and also includes a pair of upper and lower dampers placed between the container and the packaging box. The dampers absorb impact applied from outside, whereby the container and the work placed therein are prevented from being damaged.
2. Description of the Related Art
FIG. 4 shows a known damper system for transporting semiconductor wafers or recording disks. The known damper system includes a packaging box 1 including a corrugated fiberboard; a lower damper 2, placed on a bottom section of the packaging box 1, having a recessed retaining portion 4; a container 5, made of plastic or resin, storing several tens of semiconductor wafers or recording disks; and an upper damper 3 placed on the container 5. The container 5 is retained with the recessed retaining portion 4 in a fitted manner and placed between the lower damper 2 and the upper damper 3. The packaging box 1 is sealed with staples or a strip of adhesive tape. Therefore, the container 5 is protected from impact applied from outside during the transportation thereof and therefore prevented from being injured or damaged, and the semiconductor wafers or the recording disks placed in the container 5 are also prevented from being injured or damaged.
The lower and upper dampers 2 and 3 principally include a molded article made of a thin plastic sheet; a foam-molded article made of polyethylene, polypropylene, or polyurethane; or a laminated corrugated fiberboard. Those materials can be readily distorted by impact and cannot be readily restored to their former state; hence, the lower and upper dampers 2 and 3, once distorted, cannot absorb impact continuously applied thereto.
Japanese Unexamined Patent Application Publication No. 2000-208602 discloses a shipping box that is partly reusable and hardly propagates impact. The shipping box is made of metal and includes a cushioning material placed therein. However, the shipping box is not suitable for practical use because the usage cost is high unless the box is fully reused.
On the other hand, large-diameter semiconductor wafers with a diameter of 300 mm have a weight at least twice greater than that of known semiconductor wafers with a diameter of 200 mm. Therefore, containers for storing the large-diameter semiconductor wafers have a large weight. Increase in the diameter of semiconductor wafers causes a decrease in the strength of the wafers; hence, a slight impact that does not cause damage in the 200 mm wafers during the transportation causes serious damage in the 300 mm wafers during the transportation in some cases.
For example, Japanese Unexamined Patent Application Publication No. 2002-160769 discloses a bellows, which is one of dampers for containers that do not cause an increase in the volume or size of packages and absorb impact, the bellows being made of a molded sheet containing polypropylene or polyethylene. However, the bellows has an insufficient ability to absorb impact and cannot therefore be used for transporting the 300 mm wafers.
In general, molded plastic sheets and laminated corrugated fiberboards used in known techniques have low resilience. Therefore, once the sheets and the fiberboards receive a strong impact, they are distorted and cannot function as dampers. Dampers made of foamed polyurethane can be restored to its former state if the dampers receive impact. However, in the dampers, there is a problem in that poisonous gas is generated when the dampers discarded are burned.
The inventors have investigated the ability to absorb impact, the resilience, and the disposal problem, found that foamed polyethylene and polypropylene are the best materials for forming dampers, and then developed a new type of damper that is suitable for transporting a large-diameter semiconductor wafer with a diameter of 300 mm.
Dampers containing foamed polyethylene or polypropylene will now be described with reference to FIGS. 4, 5, and 6. FIG. 5a is a plan view showing an upper damper 3 and FIG. 5b is a sectional view showing the upper damper 3 taken along the line C-C′ of FIG. 5a. FIG. 6a is a plan view showing a lower damper 2 and FIG. 6b is a sectional view showing the lower damper 2 taken along the line C-C′ of FIG. 6a and a portion of the lower damper 2 in an enlarged manner.
The upper damper 3 includes eight first impact-absorbing sections 6 and four upper impact-absorbing sections 7. The front, rear, right, and left faces of the upper damper 3 each have two of the eight first impact-absorbing sections 6 thereon and the upper face of the upper damper 3 has the upper impact-absorbing sections 7 each placed at corresponding predetermined regions (regions each corresponding to the corners of a container 5) each located close to the four corners of the upper face. When impact is applied to the upper damper 3 in a direction, some of the first impact-absorbing sections 6 and upper impact-absorbing sections 7 are pressed and thereby distorted, whereby the applied impact is absorbed. The upper damper 3 has a container-retaining section, lying over the container 5 with a size specified in the SEMI standard (M31), for retaining each of various types of containers. The lower damper 2 as well as the upper damper 3 include eight second impact-absorbing sections 9 and four lower impact-absorbing sections 10. The front, rear, right, and left faces of the lower damper 2 each have two of the eight second impact-absorbing sections 9 thereon and the lower face of the lower damper 2 has the lower impact-absorbing sections 10 each placed at corresponding predetermined regions (regions each corresponding to the corners of the container 5) each located close to the four corners of the lower face. When impact is applied to the lower damper 2 in a direction, some of the second impact-absorbing sections 9 and lower impact-absorbing sections 10 are pressed and thereby distorted, whereby the applied impact is absorbed.
The upper and lower impact-absorbing leg sections 7 and 10 have the same height and shape, trapezoidal, the height of the upper impact-absorbing leg sections 7 being represented by h1 and that of the lower impact-absorbing leg sections 10 being represented by h2. Therefore, when a strong impact is applied to the lower and upper dampers 2 and 3 in the lower direction, the lower and upper dampers 2 and 3 cannot sufficiently absorb the impact.